As an apparatus for polishing a surface of a workpiece such as a silicon wafer, there are a single-side polishing apparatus, in which the workpiece is polished by each side, and a double-side polishing apparatus, in which the both surfaces of the workpiece are polished at the same time.
As shown in FIG. 9, for example, a common single-side polishing apparatus comprises a turn table 93 onto which a polishing pad 94 is attached, a polishing agent supply mechanism 96, a polishing head 92 and so on. This polishing apparatus 91 polishes a workpiece W by holding the workpiece W with the polishing head 92, supplying a polishing agent 95 onto the polishing pad 94 through the polishing agent supply mechanism 96, rotating both the turn table 93 and the polishing head 92, and bringing the front surface of the workpiece W into sliding contact with the polishing pad 94.
With high integration of electronic devices in recent years, optical lithography processes are performed on silicon wafer substrates with smaller features, and minimum line widths are less than or equal to 45 nm. The shrinking of feature sizes is being advanced. There is therefore an increasing need for highly flattened silicon wafers.
The flatness of a workpiece such as a silicon wafer is finally determined in a single-side polishing process. In this process, as a method for holding the workpiece to improve the flatness, there is a method of attaching the workpiece onto a flat disk-shaped plate having high rigidity through an adhesive such as a wax. Particularly when uniform polishing stock removal is needed for the entire surface of the workpiece, a so-called rubber-chuck method is used in which a rubber film is used as a workpiece-holding portion instead of the disk-shaped plate having high rigidity, a pressurized fluid such as air is poured into the back face of the rubber film, and the rubber film is inflated by a uniform pressure so as to press the workpiece toward the polishing pad (See Patent Document 1, for example).
An example of the structure of a conventional polishing head by the rubber-chuck method is schematically shown in FIG. 5. An essential structure of the polishing head 101 includes an annular rigid ring 103, a rubber film 102 adhered to the rigid ring 103, and a mid plate 104 joined to the rigid ring 103. A sealed space 106 is defined by the rigid ring 103, the rubber film 102, and the mid plate 104. An annular template 105 is provided concentrically with the rigid ring 103 in a peripheral portion on a lower face portion of the rubber film 102. The pressure of the space is adjusted, for example, by supplying a pressurized fluid with a pressure adjustment mechanism 107 in the center of the mid plate 104. A pressing means, not shown, for pressing the mid plate 104 in the direction of the polishing pad 132 is provided.
With the polishing head 101 configured as above, the workpiece W is held on the lower face portion of the rubber film 102 through a backing pad 108, an edge portion of the workpiece W is held with the template 105, and the workpiece W and the template 105 are both brought into sliding contact with the polishing pad 132 attached onto the upper face of the turn table 133 by pressing the mid plate 104, thereby performing a rough polishing process.
Fine particles having a diameter of at least 45 nm or more present on the surface of silicon wafers may cause deterioration of product yields of electronic devices. A final polishing process, which is a single-side polishing process in a final stage, is closely associated with particle quality. In order to reduce particles on the surface of a workpiece such as a silicon wafer in the final polishing process, a workpiece holding plate is coupled to a polishing head body through a diaphragm while avoiding any contact of a polishing pad for final polishing except with the workpiece and a load is applied to the workpiece holding plate by using a fluid and the like, thereby performing the final polishing process (See Patent Document 2, for example).
An example of the structure of a conventional polishing head in use for final polishing is schematically shown in FIG. 6(A). In an essential structure of the polishing head 121 as shown in FIG. 6(A), a holding plate 124 having a rigid body such as ceramics is coupled to a head body 123 through a diaphragm 129 made of rubber. A large number of through-holes 130 for vacuum chuck of the workpiece W are formed in the holding plate 124. A backing pad 128 is adhered to the side on which the workpiece W is attached by chuck, and a back plate 122 is provided at a back face side. A ring 125 is also provided so as to surround the holding plate. Adjusting pressure of a space between the holding plate 124 and the back plate 122 through a passage 127 for controlling workpiece chuck enables the vacuum chuck attachment and detachment of the workpiece W. Adjusting pressure of a space between the back plate 122 and the head body 123 through a passage 126 for pressing a workpiece enables adjustment of pressing force of the workpiece W against the polishing pad 132 attached onto the turn table 133.
FIG. 6(B) shows an enlarged view around the ring 125 shown in FIG. 6(A). As shown in FIG. 6(B), polishing can be performed such that the ring 125 does not press the polishing pad 132. When the thickness of a wafer W is 0.775 mm, for example, the ring 125 and the holding plate 124 are placed so that the surface (the surface to be polished) of the wafer W protrudes from the lower face of the ring 125 by 0.20 to 0.35 mm. This prevents the wafer W from being detached during polishing and enables polishing without pressing the polishing pad 132 by the ring 125, thereby preventing generation of particles from the ring 125.
There is however a problem in that surface irregularities of the holding plate 124 having a rigid body such as ceramics is transferred to the workpiece and the flatness thereby deteriorates.
To obtain highly flat workpieces with fewer particles, the polishing head by the so-called rubber-chuck method is commonly used in the rough polishing process, in which the rubber film is used as the workpiece-holding portion, a pressurized fluid such as air is poured into the back face of the rubber film, and the rubber film is inflated by a uniform pressure so as to press both the workpiece and the template holding the edge portion of the workpiece toward the polishing pad. In the final polishing process, the polishing head for final polishing is used in which a load is applied by using a fluid and the like to the workpiece holding plate coupled to the polishing head body through the diaphragm while avoiding any contact of the polishing pad for final polishing except with the workpiece.
As described above, two kinds of polishing heads are conventionally needed, and a larger polishing machine including both of the above polishing apparatus 141 having the polishing head 101 for rough polishing and polishing apparatus 151 having the polishing head 121 for final polishing is therefore needed, for example as shown in FIG. 7. As a result, there is a need to provide a larger space in a clean room, which has high utility cost, the cost of the polishing machine itself becomes higher, and thus there arises a high cost problem.
There is also disclosed a rubber-chuck polishing head that is seems to reduce particles by avoiding any contact of the polishing pad except with the workpiece during polishing and polish the workpiece with high flatness (See Patent Document 3).